ABS, or an anti-lock brake system is a good feature to have on your car,
especially if you are not trained in emergency avoidance procedures. Most
of us are not. ABS allows you to step on the brakes as hard as you can and
still retain steering control of your car.
ABS will not make your car stop in less distance than you could stop it
without ABS, if you are competent driver of your car.
Why is all this true? An emergency occurs and you must stop your car as
quickly as possible. You must slow your car immediately and you must
retain control, perhaps steering around an obstacle or steering to miss
hitting something.
The old days, pre ABS, a typical panic reaction is to tromp on the brakes
as hard as possible, locking all four wheels, and slide to a stop. You
stop quickly, but you cannot steer a wheel that will not rotate. You steer
but the car does not respond.
Physics, especially applied to modern tires, shows that "rolling" friction,
just prior to lock up, is greater than sliding friction. Your tire will
slow you more quickly if it is turning slightly than if it is totally
locked up. (I have measured 1.25 gees deceleration, autocross tires, just
prior to lock up, and .85 gees with the front wheels locked.)
So a better reaction would be to modulate the brakes to the point just
prior to lock up. This has two advantages. You can still steer and try to
avoid the obstacle and you slow more quickly.
Enter ABS, and the reason for this note. ABS is a crutch for people who do
not know how to modulate their brakes or who panic in an emergency
situation. If you do not get to practice hard braking in slippery
conditions, or you have not tried to brake at the maximum and still steer,
or you have not had simulated accident avoidance situations, you will
probably not react correctly in an emergency situation. ABS allows you to
do the next best thing and just tromp on the brakes! It will sense wheel
lock up and unlock them for an instant to allow you to retain steering
control.
But please understand, you can do better! You can brake hard, just prior
to the point of lock up, and the brakes will work at maximum force They
will not have to slide, rotate, slide, rotate, albet at high rates, thus
reducing efficiency, even slightly.
My goal in writing this is to educate. I do not believe ABS should be
relied upon to save you in a bad situation. With a little practice you can
do better! And it may still save you if you do panic, so it is good.
I have experience with the three channel system on the 1986 Corvette, the
first year it was offered. I have experience with a two channel system
controlling the rear wheels on a Mazda minivan and with the four channel
system on the Accura NSX, the first four channel production system. In all
cases, I am able to stop faster, with practice. The Corvette’s three
channel is exceptional and extends the braking distance only slightly. The
NSX system, on the flagship car of the company, is poor, causing the car to
crab down the road as the individual wheels lock and unlock, greatly
extending the distance. The Mazda system is there to keep you from
pirouetting down the road.
I was going to suggest practicing accident avoidance, but my lawyer would
have a fit! I autocross, a low speed, controlled environment, safe. I
also have been known to go to an empty parking lot after a rain. What I do
there, I have a hard time remembering…
Michael Sands Kaleida Labs, Inc.
sa…@kaleida.com 415.966.0711